Woman Diagnosed With Mosaic Down Syndrome at 23 After Years of Unanswered Questions

Ashley Zambelli lived most of her life unaware that a genetic link tied together some of her health issues and family history. It all changed when she was 23, after a few genetic tests related to pregnancies prompted doctors to check her own chromosomes more closely. She finally got the diagnosis: mosaic Down syndrome, a condition in which not all cells carry the extra chromosome.

The Pregnancy Clues That Led Doctors to Look Deeper

What is striking, though, is that Zambelli did not begin her journey to diagnosis with any routine testing on herself. In 2019, genetic testing had allegedly shown that a pregnancy involved Trisomy 21 following a miscarriage. She later had a child who was born with Down syndrome and then another daughter who did not have it. At a later gestation, screening again suggested an increased risk of Trisomy 21.

Given how unusual multiple pregnancies with Trisomy 21 are for a woman in her early twenties, her doctors speculated as to whether there could be a genetic basis. Later, her reproductive history turned out to be the clue that pushed specialists to study her own chromosomes further.

As MedlinePlus Genetics explains, Down syndrome is a chromosomal disorder caused by an extra copy of genetic material on chromosome 21. The exact genetic change may vary depending on which type of Down syndrome is involved.

Why Her Blood Tests Failed to Reveal the Answer

It was a tricky testing process. As Zambelli explained, she had karyotype and FISH testing on blood samples, but those tests returned negative. Even though doctors suspected some sort of mosaicism, the chromosomes seen in her blood cells looked normal.

The investigation subsequently moved to a buccal smear, which picks up cells from within the cheek. Testing on those cells found that a few were affected by Trisomy 21, so mosaic Down syndrome was diagnosed, Zambelli said. About 3% of the cells studied had the extra chromosome, she said.

According to the National Down Syndrome Society, mosaicism is a combination of two types of cells. Some cells have the expected 46 chromosomes, while others have 47, with an extra chromosome 21.

This disparity between cell populations may account for some of the difficulty in detecting a genetic condition related to mosaicism from a single biological sample.

Why Mosaic Down Syndrome Can Be Easy to Miss

A certain degree of physical similarity can be associated with Down syndrome, but this does not apply to everyone. Zambelli has said she does not have many of the facial features people frequently associate with the condition, something that has occasionally prompted others to question her diagnosis.

Long before she had any inkling that there might be a genetic reason for these health and learning issues, she had already experienced them. We are told these included patellar dislocations, problems with the jaw, low muscle tone, memory issues, tachycardia, and even difficulty doing well in school-specific testing.

These worries had seemed disconnected in the past, but the genetic diagnosis provided her with a potential rationale for why some things had happened throughout her life.

One crucial lesson from this experience is that there is no single form, health status, or functioning profile shared by all individuals with mosaic Down syndrome. The degree or prevalence of affected cells can differ between individuals.

NHS guidance on Down syndrome also states that the condition is caused by an extra chromosome, and people with Down syndrome are not all the same. They can have different health requirements, abilities, and levels of independence.

What Makes Mosaic Down Syndrome Different?

There are three major genetic forms of Down syndrome. The most common of them, according to the Centers for Disease Control and Prevention, is Trisomy 21, where each cell usually has three copies of chromosome 21.

In translocation Down syndrome, a part or whole additional chromosome 21 is attached to another chromosome.

It is the mosaic form that is rarer. In this form, some cells in a person have the extra chromosome while other cells contain the standard number of chromosomes. Due to the fact that the proportion of affected cells can differ, traits can vary greatly between different individuals.

Such differences are one of the reasons why doctors may be unable to identify or confirm this condition through appearance alone.

Chromosome testing and other tissue types are sometimes used for medical evaluation. But it would be wrong for individuals to assume they have a chromosomal condition based on their symptoms or any obvious features.

A medical geneticist or genetic counselor can decide whether genetic testing is appropriate based on family history and previous test results.

The Diagnosis That Finally Connected the Dots

For Zambelli, getting a diagnosis was more than putting a label on a genetic finding. She has said that she was glad to finally learn the reasons, as it “draws a connection between health and learning issues that previously seemed separate.”

She has also publicly discussed people telling her that she does not “look” like someone who has Down syndrome. Her case works against the common view that any genetic condition should be evident in childhood.

For her, the diagnosis only arrived after further information was gleaned from her pregnancy history and specialists ordered more tests.

Her story is also an illustration of why mosaic Down syndrome can look different in different people and how visual observation alone cannot yield a reliable diagnosis.

Overall, Zambelli has a wider message to deliver about genetic health. Chromosomal conditions can affect people very differently, and visible features do not tell the whole story.

If there is a real medical rationale for investigation, professional assessment, including appropriate genetic testing, yields answers far more valid than guesses based simply on looking at someone.